Redox flow batteries are used for measures for electric load-leveling and momentary stop, and are attracting attention as novel batteries for electricity storage. For example, a redox battery in which vanadium is used as an active material is known (see Patent Document 1, for example). Patent Document 1 discloses a redox flow battery in which a dimensionally stable electrode including a titanium sheet or a titanium mesh plated with a noble metal, a graphite rod, a graphite plate, or a carbon fiber material is used as a positive electrode material. Regarding the redox flow battery disclosed in Patent Document 1, conduction failure due to anode passivation of the titanium sheet, and insufficient durability due to oxidation of the graphite or carbon material are described. Further, Patent Document 1 describes the corrosion resistance of an electrode composed of titanium whose surface is covered with iridium oxide, but is silent about the cycle performance of the battery including the electrode.
In recent years, a carbon felt material is often used as the electrode material for the positive electrode and negative electrode in a redox flow battery, and in many cases, the battery is operated with limitations on the charging voltage in order to improve the durability (see Patent Document 2, for example).
In general, a vanadium redox flow battery includes a separation membrane composed of an ion exchange membrane, and a felt positive electrode and a felt negative electrode made of a carbon material and disposed on both sides of the separation membrane. A positive electrode chamber and a negative electrode chamber are disposed so as to incorporate the respective electrodes and electrolytes therein, thereby forming a single cell battery. In the case of a cell stack battery in which a plurality of single cells are combined, bipolar plates each configured to collect power from adjacent positive electrode and negative electrode are disposed between the respective cells. Further, the above described redox flow battery is provided with a positive electrode electrolyte tank for storing a positive electrode electrolyte and a negative electrode electrolyte tank for storing a negative electrode electrolyte, and the positive electrode electrolyte is supplied into the positive electrode chamber by a positive electrode electrolyte pump to initiate a positive electrode reaction at the surface of the felt positive electrode, and the negative electrode electrolyte is supplied into the negative electrode chamber by a negative electrode electrolyte pump to initiate a negative electrode reaction at the surface of the felt negative electrode to carry out discharge, thereby allowing for extraction of electricity outside the battery. On the other hand, charging is carried out by energizing the electrodes from the outside while allowing the positive electrode electrolyte and the negative electrode electrolyte to flow into the positive electrode chamber and the negative electrode chamber, respectively. The formulas representing the reactions at the positive electrode and the negative electrode are as follows.
Positive Electrode:V4+→V5++e− (charging) V4+←V5++e− (discharging)
Negative Electrode:V3++e−→V2+ (charging) V3++e−←V2+ (discharging)
In reality, however, it is assumed that V4+ is present as VO2+, and V5+ is present as VO2+, each in a hydrated state or in a state coordinated with sulfate radical.
The vanadium redox battery as described above has the following advantageous properties as compared to other batteries.
1. The battery is operable at room temperature.
2. The battery is extremely stable and has a long cycle life.
3. The battery has no explosive/flammable properties due to containing no hazardous material.
4. The amount of electricity storage in the battery can be easily increased, due to its active material being liquid and the liquid being stored in a tank.
5. The state of charging and discharging of the battery can be controlled by observing the composition of the electrolytes.
6. The battery can be easily regenerated even if a plurality of ions exists in mixture.
However, the energy density of the vanadium redox flow battery is lower as compared to other secondary batteries, since the saturated dissolved concentration of the vanadium active material is not so high, and the cell voltage of the single cell is not so high due to the electrolyte being an aqueous solution. At the same time, although the vanadium redox flow battery has a significantly high instantaneous output, it has a practical current density of about several 10 mA/cm2, with 200 mA/cm2 being the upper limit, because the internal resistance of the battery, particularly the flow resistance of the electrolyte inside the electrodes, is too high to continuously carry out charging and discharging at a high current. One of the causes for this is the resistance against the flow of the electrolyte in the carbon felt used for the electrodes. However, if the felt density is decreased in order to reduce the resistance, the electron conductivity is reduced. In other words, they are in a trade-off relationship.